Apparatus for recording image including an afocal optical system

ABSTRACT

Laser beams corresponding to an image are directed through an afocal optical system such that chief rays of the laser beams outgoing from the system are substantially perpendicular to a recording surface and such that beam waists of the laser beams are approximately located at the recording surface. Thus, the image can be recorded on the recording surface with high picture quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus, such as a laser plotteror a graphic arts color scanner, for recording an image. In particular,the present invention relates to an image recorder which scans an imagerecording surface with a plurality of parallel recording beams.

2. Description of the Prior Art

FIG. 1 is a perspective view showing a conventional laser plotter orgraphic arts color scanner. The image recorder includes a recording headA and a driving unit (not shown) for moving the recording head A in theX-direction.

In operation, the laser beam B₁ from a light source 50 enters a beamexpander 52, its diameter is expanded, and the expanded laser beam B₂ isprojected onto a stop 54 having an aperture 54a. A central portion B₃ ofthe expanded laser beam B₂ is projected through the aperture 54a. Theaperture 54a sharpens the edges of the image to be formed.

The laser beam B₃ which passes through the stop 54 is divided into aplurality of laser beams B₄ by a beam splitter 56. The laser beams B₄are individually modulated by an optical modulator system 58 in responseto image signals.

A multibeam B₅ from the optical modulator system 58 is reflected byreflecting mirrors 60 and 62, and then projected through a slit plate 66toward a telecentric lens system 64 and toward a photosensitive materialF which is wound on a rotating cylinder 68. Thus, the image is recordedon the photosensitive material F. The laser beams B₅ are focused by thelens system 64 onto the photosensitive material F. Therefore, theconventional image recorder is advantageous in that the size of theimage is not varied even if the distance between the rotating cylinder68 and the recording head A is changed.

However, since only the central portion of the expanded laser beam B₂passes through the aperture 54a and since the extracted laser beam B₃ isdivided into a plurality of laser beams by the beam splitter 56, a highpower light source 50 has generally been needed to be sure that thebeams B₄ are sufficiently intense. But this increases the size of theimage recorder. This problem is also present when the light source isformed by laser diodes or the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image recorderwhich can record an image on a recording surface with high picturequality.

Another object of the present invention is to reduce the size of theapparatus.

The present invention relates to an apparatus for recording an image ona recording surface, which includes: a light source for generating aplurality of recording laser beams; a modulator for individuallymodulating the laser beams in response to image signals which arerepresentative of an image; and an afocal optical system for directingthe laser beams toward a recording surface such that chief rays of eachof the laser beams are substantially perpendicular to the recordingsurface at the recording surface and such that first beam waists of thelaser beams are located approximately at the recording surface.

These and other objects, features and aspects of the present inventionwill become more apparent from the following detailed description of thepresent invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional image recorder;

FIGS. 2 and 3 are schematic diagrams of an optical system which isformed by an afocal optical system;

FIG. 4A illustrates the relationship between a beam waist of a laserbeam which is incident upon a lens and a beam waist of a laser beampassing through the lens;

FIG. 4B illustrates the relationship between a beam waist of a laserbeam which is incident upon the optical system shown in FIG. 3 and abeam waist of a laser beam passing through the optical system;

FIG. 5 illustrates the distribution of the intensity of a laser beamaround a beam waist;

FIGS. 6 and 7 are schematic diagrams of an optical system which isformed by two afocal optical systems;

FIG. 8 is a schematic diagram of an optical system which includes a zoomsystem;

FIG. 9 is a schematic diagram of an afocal optical system which isformed by an objective lens system having two lenses and an eyepiecelens system having three lenses;

FIG. 10 is a block diagram of an image recorder according to the presentinvention;

FIG. 11 is a perspective view of a recording head of an image recorderaccording to a first preferred embodiment of the present invention;

FIG. 12 illustrates a beam diameter converter;

FIG. 13 is a sectional view of a beam splitter; and

FIG. 14 is a perspective view of a recording head of an image recorderaccording to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. Principle of the Invention

Referring to FIG. 2, an afocal optical system 2 comprises two convexlenses (an objective lens L₁ and an eyepiece lens L₂) which are spacedapart from each other by the sum of their focal lengths, i.e., f₁ +f₂.

It is known from Kogaku Gijutsu Contact, Vol. 22, No. 7, 1984, KogakuKogyo Gijutsu Kyokai, p. 61, for example, that: (1) when a beam waist W₁of a laser beam B₁₁ is set at the front focal point F₁ of the objectivelens L₁, a beam waist W₂ of a laser beam B₂₁ is formed at the back focalpoint F₂ of the eyepiece lens L₂ and (2) the radii ω₁ and ω₂ of thelaser beams B₁₁ and B₂₁ at the beam waists W₁ and W₂ satisfy thefollowing relationship: ##EQU1## where m is the magnification of thesystem 2.

The term "beam waist" refers to the point where the width of a laserbeam is narrowed to the minimum. The radius of curvature of a wave frontis infinite at the beam waist.

Noting only the chief ray of a laser beam, it is known that a laser beampropagates according to the theories of geometric optics. Therefore,when parallel laser beams B₁₁ and B₁₂ having a beam pitch P₁ areprojected onto the objective lens L₁, parallel laser beams B₂₁ and B₂₂having a beam pitch P₂ outgo from the eyepiece lens L₂. The relationshipbetween P₁ and P₂ is as follows: ##EQU2##

It is understood from equations (1) and (2) that the radii of the beamsB₂₁ and B₂₂ at the beam waists and the pitch of parallel laser beamspassing through the afocal optical system 2 are proportional to themagnification m (=f₂ /f₁) of the system 2.

In FIG. 3, the beam waists of the laser beams B₁₁ and B₁₂ are not set atthe front focal point F₁ of the objective lens L₁. Instead, the beamwaists are set at a point W₁. Symbol ΔZ₁ represents the distance betweenthe front focal point F₁ and the beam waist W₁. Symbol ΔZ₂ representsthe distance between the back focal point F₂ of the eyepiece lens L₂ andthe beam waist W₂ of the laser beams B₂₁ and B₂₂. The absolute values ofthe distances ΔZ₁ and ΔZ₂ are proportional to the square of themagnification m. The reason for this is explained in connection withFIGS. 4A and 4B as follows:

When a laser beam B (TEM₀₀ mode beam) being in Gaussian distribution isconverged by a lens L having a focal length f, the optical system shownin FIG. 4A satisfies the following relationship: ##EQU3## where symbolZ_(F) represents the distance between a beam waist W_(F) of an incidentlaser beam B and the lens L, symbol Z_(R) represents the distancebetween the lens L and a beam waist W_(R) of a laser beam B' outgoingfrom the lens L, symbols ω_(F) and ω_(R) represent beam radii at thebeam waists W_(F) and W_(R), respectively, and symbol λ represents thewavelength of the laser beam B (B').

As shown in FIG. 4B, therefore, the objective lens L₁ of the afocaloptical system 2 satisfies the following relationship: ##EQU4## wheresymbol Z_(F1) represents the distance between the beam waist W₁ and theobjective lens L₁, symbol Z_(R1) represents the distance between theobjective lens L₁ and a beam waist W₁₂ of a laser beam outgoing from thelens L₁, and symbol ω_(F1) represents a beam radius (=ω₁) at the beamwaist W₁ .

Similarly, the eyepiece lens L₂ of the afocal optical system 2 satisfiesthe following relationships: ##EQU5## where symbol Z_(F2) represents thedistance between the beam waist W₁₂ and the eyepiece lens L₂, symbolZ_(R2) represents the distance between the eyepiece lens L₂ and the beamwaist W₂ of the laser beam outgoing from the lens L₂, and symbol ω_(F2)represents a beam radius (=ω₂) at the beam waist W₂.

From FIG. 4B, assuming distances ΔZ₁ and ΔZ₂ are negative when the beamwaists W₁ and W₂ are closer to the lenses L₁ and L₂, respectively, therelationship between the distances Z_(F1), Z_(R2) and the focal lengthsf₁, f₂ are as follows:

    Z.sub.F1 =f.sub.1 +ΔZ.sub.1                          . . . (9)

    Z.sub.R2 =f.sub.2 +ΔZ.sub.2                          . . . (10 )

Since the optical system 2 is an afocal system, the optical system 2satisfies the following relationship:

    Z.sub.R1 +Z.sub.F2 =f.sub.1 +f.sub.2                       . . . (11)

It then follows from equations (5) to (11) that the relationship betweenthe distance ΔZ₁ and the distance ΔZ₂ is as follows: ##EQU6## Differentcoordinate systems are utilized in equation (12') for the front and rearfocal sides of the afocal system for calculating an absolute value (seeFIG. 4B).

With measurement in a common coordinate system, the relationship betweenthe distances ΔZ₁ and ΔZ₂ is as follows: ##EQU7##

It is appreciated from the equation (12) that the distances ΔZ₁ and ΔZ₂are proportional to the square of the magnification m of the afocaloptical system 2.

Also, a beam pitch P₂ of the laser beams B₂₁ and B₂₂ and a beam radiusω₂ at the beam waist W₂ are obtained from the equations (5) and (11) andexpressed by the above equations (1) and (2), respectively. Thus, it isnoted that the beam pitch P₂ and the beam radius ω₂ are not functionallyrelated to the distances ΔZ₁ and ΔZ₂.

Thus, when a recording surface is located at the beam waist W₂,recording can be made by laser beams whose beam radii correspond to thebeam radius ω₂.

FIG. 5 illustrates the distribution of the intensity of a laser beamaround a beam waist. In this figure, light intensity is measured alongthe X-axis, beam diameter is measured along the Y-axis, and the distancefrom the beam waist is measured along the Z-axis. As is apparent fromFIG. 5, by locating the recording surface at the beam waist, it ispossible to record a sharp image on the surface.

FIG. 6 illustrates an optical system which is provided with the afocaloptical system 2 and another afocal optical system 4. The system 4includes two convex lens L₃ and L₄ which are spaced apart from eachother by the sum of their focal lengths, i.e., f₃ +f₄. The distancebetween the lens L₂ and the lens L₃ is equal to f₂ +f₃.

Assuming that symbol ΔZ₄ represents the distance between a beam waist W₄of laser beams B₄₁ and B₄₂ outgoing from the eyepiece lens L₄ and a backfocal point F₄ of the eyepiece lens L₄, the relationship between thedistance ΔZ₄ and the distance ΔZ₁ is as follows: ##EQU8##

Similarly, the relationship between a beam pitch P₁ of the laser beamsB₁₁ and B₁₂ and a beam pitch P₄ of the laser beams B₄₁ and B₄₂ is asfollows: ##EQU9##

Further, the relationship between a beam radius ω₁ of the laser beamsB₁₁ and B₁₂ at the beam waist W₁ and a beam radius ω₄ of the laser beamsB₄₁ and B₄₂ at the beam waist W₄ is as follows: ##EQU10## FIG. 7 showsanother optical system in which the beam waist W₂ of the laser beamsoutgoing from the afocal optical system 2 coincides with the front focalpoint F₃ of the objective lens L₃ of the afocal optical system 4, i.e.,the distance between the lenses L₂ and L₃ is f₂ +f₃ -ΔZ₂.

In the optical system shown in FIG. 7, the beam waist W₄ of the laserbeams B₄₁ and B₄₂ outgoing from the eyepiece lens L₄ coincides with theback focal point F₄ of the eyepiece lens L₄ since (as in FIG. 2) thefront focal point F₃ of the objective lens L₃ coincides with the beamwaist W₂ of the incident laser beams. The beam pitch P₄ of the laserbeams B₄₁ and B₄₂ and the beam radius ω₄ of the laser beams B₄₁ and B₄₂at the beam waists thereof are determined by equations (14) and (15),respectively.

A sharp image can be obtained by arranging a recording surface at thebeam waist W₄ of the laser beams B₄₁ and B₄₂ outgoing from the opticalsystems shown in either FIG. 6 or FIG. 7.

Since the optical systems of FIGS. 6 and 7 are formed of two afocaloptical systems 2 and 4, magnification may be controlled by flexiblystructuring the afocal optical systems 2 and 4 while aberration can bedecreased as compared with the single-stage optical system shown inFIGS. 2 and 3.

FIG. 8 shows an optical system which includes a zoom system L₅. Thesystem illustrated in FIG. 8 is otherwise similar to the systemillustrated in FIG. 7. The zoom system L₅ includes two convex lenses L₅₁and L₅₃ and a concave lens L₅₂. The zoom system L₅ is disposed betweenthe lenses L₃ and L₄. The lens L₃ and the zoom system L₅ form a combinedlens system L₆ with a focal length f₆.

The front focal point F₆ of the lens system L₆ coincides with the beamwaist W₂. Symbols H and H' represent front and rear principal points ofthe lens system L₆, respectively. The focal length f₆ changes accordingto the movement of the lenses L₅₁ through L₅₃ and the front and backprincipal points H and H' move by the same distance in a contrarydirection so that the front and back focal points of the lens system L₆do not move. Thus, the beam waist W₄ does not move when themagnification of the zoom system L₅ is changed by moving the lenses L₅₁to L₅₃.

On the other hand, the pitch of the laser beams B₄₁ and B₄₂ and the beamradius at the beam waist W₄ are proportional to the magnification of thesystem.

Thus, a sharp image can be obtained in any magnification by arranging arecording surface at the beam waist W₄ and moving the lenses L₅₁ throughL₅₃.

From the foregoing, the following observations can be made:

First, the light density of a laser beam is highest at its beam waist,and the beam waist of a laser beam outgoing from the optical systemshown in each of FIGS. 2, 3, 4, 6, 7 and 8 can be determined. Thus, arecording surface can be easily located at the beam waist such that asharp image can be recorded.

Further, parallel outgoing laser beams are generated when parallel laserbeams enter the optical systems described above. The parallel outgoinglaser beams are substantially vertical with respect to the recordingsurface. Therefore, the size of the image formed on the surface does notvary when the distance between the recording surface and the opticalsystem is changed slightly.

The present invention is not limited to the optical systems described.An afocal system formed by an objective lens system having two lens L₁₁and L₁₂ and an eyepiece lens system having three lens L₂₁, L₂₂ and L₂₃,as shown in FIG. 9, may be used. In general, the objective lens systemand the eyepiece lens system are formed by a plurality of lenses tocorrect aberration.

B. First Embodiment

FIG. 10 is a block diagram of a laser plotter which includes an imagedata processor 80, a data converter 82 and an image recorder 84.

The image data processor 80 includes a minicomputer 86 for calculatingvector data of an image to be recorded, a CRT 88, a keyboard 90, amagnetic tape unit 92 and a magnetic disk unit 94 for storing CAD data.The vector data is obtained from the CAD data.

The data converter 82 converts the vector data into dot data. The imagerecorder 84 records the image on photosensitive material 26 which iswound on a rotating cylinder 24. The recording is controlled by the dotdata from the data converter 82. The image recorder 84 includes arecording head 10 and a driving unit (not shown) coupled to therecording head 10 for moving the same in the X-direction.

Referring to FIG. 11, the recording head 10 comprises a laser beamsource 12, a beam diameter converter 14, a beam splitter (dividingmeans) 16, a multi-channel optical modulator 18, a slit plate 20,mirrors 28a to 28d, an optical system 22 (structured as shown in FIG. 8)and a stepping motor 29 for moving the zoom lenses of the optical system22.

In operation, a laser beam B₁ from the source 12 enters the beamdiameter converter 14 through four mirrors 28a to 28d.

Referring to FIG. 12, the beam diameter converter 14 comprises twoconvex lenses 30 and 32. The distance between the lenses 30 and 32 issuch that a beam waist W of the reduced diameter laser beam B₂ isimmediately in front of the lens 32. The beam diameter converter 14 isprovided between the laser beam source 12 and the beam splitter 16 forthe following reason:

The optical system 22 includes the zoom system illustrated in FIG. 8 forchanging magnification. The zoom system makes it possible to change thediameter of beams outgoing from the system 22. However, when themagnification of the system is changed, the pitch of the beams outgoingfrom the optical system 22 is also changed. The beam diameter converter14 makes it possible to set the beam diameter at a prescribed valuewithout changing the beam pitch.

The function of the converter 14 is similar to that of the aperture 54ain that the converter 14 reduces the diameter of the laser beam B₂. Butunlike the aperture 54a, the converter 14 does not change thedistribution of the intensity of the light passing therethrough. Thatis, the intensity of the beam B₂ has a Gaussian distribution across itswidth.

The converter 14 is not restricted to the Keplerian type shown in FIG.12. The converter may be a Galileian or Cassegrainian type, or a focalsystem.

As shown in FIG. 13, the beam splitter 16 includes a glass plate 16awith a pair of parallel flat surfaces S1, S2. A total reflection film16b is adhered to the surface S1 and a semi-transparent film 16c isadhered to the surface S2.

An uncovered portion of the surface S1 faces the beam diameter converter14. The surface S2 is covered entirely by the semi-transparent film 16c.The film 16c includes films 161 to 164 which stepswisely transmit morelight and stepwisely reflect less light. Thus, an incident laser beam B₂which enters the glass plate 16a at a slight inclination is internallyreflected between the total reflection film 16b and the semi-transparentfilm 16c, while laser beams B₃₀, B₃₁, . . . , B₃₉ pass through thesemitransparent film 16c at regular intervals. Thus, the beam splitter16 divides the laser beam B₂ into a multibeam B₃ consisting of tenparallel laser beams B₃₀, B₃₁, . . . , B₃₉ outgoing toward the opticalmodulator 18.

The optical modulator 18 has an acoustooptic, electro-optic ormagneto-optic modulation element (not shown) for each of the beam B₃₀,B₃₁, . . . , B₃₉ so as to individually modulate the beams B₃₀, B₃₁, . .. , B₃₉ in response to image signals. The structure and operation etc.of the optical modulator 18 are well known to those skilled in the art,and hence redundant description is omitted.

The slit plate 20 is adapted to pass only primary diffracted laser beamsmodulated by the optical modulator 18. The plate 20 shields zero-orderdiffracted light.

A multibeam B₅ extracted from the slit plate 20 enters the opticalsystem 22 (i.e., the system illustrated in FIG. 8). In the opticalsystem 22, the parallel laser beams of multibeam B₅ are reduced and thendirected toward the photosensitive material 26. The rotating cylinder 24is located substantially at the rear focal point F₄ of the eyepiece lensL₄ (FIG. 8), i.e., the beam waist W₄ of the laser beams outgoing fromthe optical system 22, so that a sharp image is recorded on thephotosensitive material 26.

The laser beams outgoing from the optical system 22 are substantiallyperpendicular to the photosensitive material 26 (surface of the cylinder24). Therefore, the size of the image formed on the material 26 does notvary even if the photosensitive material 26 (or the cylinder 24) movesslightly.

Since the beam from the laser beam source 12 is never restricted by anaperture or the like, a relatively small, low power laser beam source 12can be used. Accordingly, the size of the image recorder can be reduced.

The beam splitter causes the laser beams B₃₀, B₃₁, . . . , B₃₉ to haveslightly different optical lengths. Hence, the beams B₃₀, B₃₁, . . . ,B₃₉ have slightly different optical beam waists. However, since the beamwaists of the laser beams being converged by the optical system 22 areproportional to the square of the magnification m of the optical system22 (about 1/100 to 1/300), the differences between the beam waists areso small as to be negligible.

In an alternative embodiment, the laser beam B₁ may enter the beamsplitter 16 without first being reduced.

The optical system 22 may be formed as shown in FIGS. 2, 3, 6, 7 or 8.The optical system may also be formed by three or more afocal opticalsystems.

B. Second Embodiment

FIG. 14 is a perspective view showing a recording head of an imagerecorder according to a second preferred embodiment of the presentinvention. As shown in the figure, a light source unit 100 comprises alight source group 101 having a plurality of light sources (such aslaser diodes) and a beam adjuster 102 for adjusting the shapes and beamwaists of laser beams outgoing from the light source group 101. Thelight sources of the group 101 emit laser beams in response to signalsfrom a control unit (not shown), whereby beams B₅₁ which correspond tothe image to be recorded are projected onto the laser beam adjuster 102.

A multibeam B₅₂ from the beam adjuster 102 enters the optical system 22,which is structured as shown in FIG. 8. As in the first embodiment, thebeams B₅₂ are reduced and then directed toward photosensitive material26 to record an image thereon.

In an alternative embodiment, the multibeam B₅₁ enters the opticalsystem 22 without passing through the beam adjuster 102.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of illustration. Thespirit and scope of the present invention should be limited only by theterms of the appended claims.

What is claimed is:
 1. An apparatus for recording an image on arecording surface, comprising:a light source for generating a pluralityof recording laser beams; a modulator for individually modulating saidlaser beams in response to image signals which are representative of animage; and an afocal optical system for directing said laser beamstoward a recording surface such that chief rays of each of said laserbeams are substantially perpendicular to said recording surface at saidrecording surface; wherein first beam waists of each of said laser beamscoincide with a front focal plane of said afocal optical system; andwherein said recording surface is located at a rear focal plane of saidafocal optical system.
 2. An apparatus for recording an image on arecording surface, comprising:a light source for generating a pluralityof recording laser beams; a modulator for individually modulating saidlaser beams in response to image signals which are representative of animage; and an afocal optical system for directing said laser beamstoward a recording surface such that chief rays of each of said laserbeams are substantially perpendicular to said recording surface at saidrecording surface; wherein first beam waists of each of said laser beamsdo not coincide with a front focal plane of said afocal optical system;and wherein said recording surface does not coincide with a rear focalplane of said afocal optical system; and wherein said afocal opticalsystem satisfies the following relationship:

    ΔZ.sub.R = m.sup.2 ·Δ Z.sub.F

where ΔZ_(R) is the distance between said rear focal plane and saidrecording surface, ΔZ_(F) is the distance between said front focal planeand said first beam waists, and m is the magnification of said afocaloptical system.
 3. An apparatus for recording an image on a recordingsurface, comprising:a light source, including: (1) means for generatinga source laser beam and (2) beam splitting means for splitting saidsource laser beam into a plurality of recording laser beams, each ofsaid recording laser beams having a slightly different optical length; amodulator for individually modulating said recording laser beams inresponse to image signals which are representative of an image; and anafocal optical system for directing said recording laser beams toward arecording surface, such that chief rays of each of said recording laserbeams are substantially perpendicular to said recording surface at saidrecording surface, said afocal optical system being a reduction lenssystem; wherein first beam waists of each of said recording laser beamsdo not coincide with a front focal plane of said afocal optical system;and wherein said recording surface is located at a rear focal plane ofsaid afocal optical lens system; and wherein said afocal optical systemsatisfies the following relationship:

    ΔZ.sub.R =m.sup.2 ·ΔZ.sub.F

where ΔZ_(R) is the distance between said rear focal plane and saidrecording surface, ΔZ_(F) is the distance between said front focal planeand said first beam waists, and m is the magnification of said afocaloptical system.